Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
In a typical process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with fixed beds of catalyst which receive upflow or downflow feed, and each is provided with means for preheating the feed because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or hydrogen recycle gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, i.e., a C.sub.5.sup.+ or C.sub.5 /430.degree. F. fraction, and a vaporous effluent. The latter is a gas rich in hydrogen which usually contains small amounts of normally gaseous hydrocarbons. Hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production, hydrogen being produced in net yield.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, polymetallic platinum metal catalysts have been employed to provide, at reforming conditions, improved catalyst activity, selectivity and stability. Thus, one or more additional metallic components have been added to platinum as promotors to further improve, particularly, the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, palladium, selenium, tin, copper and the like. Platinum-rhenium catalysts, for example, possess superior selectivity for use in reforming operations as compared with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
Platinum-rhenium catalysts have been staged in the reactors of reforming units in various ways in order to improve the overall activity, or selectivity of the catalyst. For example, in application Ser. Nos. 082,804 and 082,805 by Swan and Oyekan and Swan, respectively, filed Sept. 9, 1978, the lead reactors are charged with low rhenium platinum-rhenium catalysts, or catalysts wherein the atomic ratio of rhenium:platinum is 1:1, or less, and the tail reactor, or last reactor of the reactor series contains a high rhenium, platinum-rhenium catalyst, or catalyst wherein the atomic ratio of rhenium:platinum is at least 1.5:1, and preferably 2:1 and greater. Higher C.sub.5.sup.+ liquid yield is obtained than in the more conventional use of platinum-rhenium catalysts wherein all of the reactors of a unit contain a low rhenium, platinum-rhenium catalyst; or in accordance with U.K. Patent Application GB No. 2 028 278A wherein all of the reactors of a unit contain a high rhenium, platinum-rhenium catalyst. Pressure has also been found to affect the reforming operations employing such catalysts. At ultra low pressure conditions (e.g., 175 psig, 3000 SCF/B hydrogen recycle) it was found, and disclosed in application Ser. No. 409,073 (OP-2871), filed Aug. 18, 1982, by William E. Winter, Jr. and Gerald E. Markley that both catalyst activity and yield stability were increased even with amounts of high rhenium, platinum-rhenium catalysts greater than disclosed in the Ser. Nos. 082,804 and 082,805 application, supra, distributed throughout the rearwardmost reactors of a reforming unit; even when all of the reactors of the unit were charged with a high rhenium, platinum-rhenium catalyst as disclosed in U.K. Patent Application GB No. 2 018 278A, supra. In fact, in the unit wherein all of the reactors were charged with a high rhenium, platinum-rhenium catalyst the superior yield stability was demonstrated by a C.sub.5.sup.+ liquid yield credit of 0.8 LV% after 1000 hours on-oil. However, the high cycle average C.sub.5.sup.+ yields of such system were significantly compromised by a period of excessive cracking experenced at, and near the start of a run. This excessive cracking, a phenomenon known as hydrogenolysis wherein there is excessive gas make and loss of C.sub.5.sup.+ liquid yield, has commonly been observed at start-of-run conditions with rhenium-containing catalysts. At start-up the production of C.sub.1 -C.sub.4 gases commences, and gradually decreases with concurrent increase in the production of C.sub.5.sup.+ liquids. Eventually the production of C.sub.1 -C.sub.4 gases levels off and the C.sub.5.sup.+ liquid yield lines-out which marks the end of the start-up period. Although the cracking phenomenon is usually temporary, it reduces start-of-run yields and adversely impacts on average cycle yields; at least proportionate with the degree and duration of the cracking behavior.
The activity of the catalyst gradually declines due, at least in part, to the build-up of coke. Coke formation is believed to result from cracking and polymerization reactions; perhaps from the deposition of coke precursors such as anthracene, coronene, ovalene and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by coke deposition. Eventually, however, economics dictates the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by removal of the coke from the catalyst. Typically, in the regeneration, the coke is burned from the catalyst at controlled conditions. In a regeneration of this type, the coked catalyst is contacted with oxygen at flame front temperatures ranging about 800.degree. F. to about 1050.degree. F., this being generally followed by a secondary burn with increased oxygen concentrations as coke is depleted from the catalyst.
Two major types of reforming are generally practiced in the multi reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series.